Repurchase Agreements Remaining Term to Maturity (Details) $ in Thousands	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Balance	$ 2,613,892	$ 2,565,902
WAVG Original Term to Maturity	46	54
Less than 30 days
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Balance	$ 2,339,638	$ 2,240,791
WAVG Original Term to Maturity	51	49
30 to 90 days
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Balance	$ 274,254	$ 274,231
WAVG Original Term to Maturity	8	90
Maturity Greater than 90 Days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Balance	$ 0	$ 50,880
WAVG Original Term to Maturity	0	121